Solvent extraction of impurities from concentrated metal sulphate solutions

ABSTRACT

The present invention relates generally to a two step solvent extraction circuit to remove impurity metals of zinc and cobalt selectively from a valuable metal of nickel. In order to selectively extract zinc there must be sufficient separation between zinc and cobalt in the cyanex  272  system. Similarly for cobalt and nickel, the separation factor must be of sufficient magnitude to obtain a pure nickel product. The process for the solvent extraction of impurity metals is operated at a temperature exceeding 60° C. being the maximum temperature at which solvent extraction circuits conventionally operate.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a method of removingimpurity metals from an impure valuable metal sulphate stream in asolvent extraction circuit. More particularly, the invention relates tothe extraction of cobalt and/or zinc from a concentrated nickel sulphateliquor.

BACKGROUND TO THE INVENTION

[0002] In the processing of nickel laterites, nickel and cobalt areselectively separated from a wide range of impurities by mixed (Ni/Co)sulphide precipitation. This mixed sulphide is then re-leached usingtemperature and oxygen over pressure. The resulting leach solution isacidic and usually has concentration ranging from 60-120 g/L nickel and5-15 g/L cobalt, together with other impurities like copper and zinc.Similarly concentrated leach solutions can also be obtained from thepressure leaching of nickel sulphide concentrates with oxygen. Afterneutralisation of the acid cobalt can then be separated from nickel viasolvent extraction. Cyanex 272 is typically used to selectively extractcobalt over nickel in a sulphate matrix at a fixed pH. However theextraction of cobalt is a cation exchange reaction which releasesprotons from the reagent which must be neutralised as this extraction ispH sensitive. Ammonia is typically used as the neutralising solutionboth to raise the pH prior to solvent extraction, and to maintain aconstant pH during extraction. The main problem with the use of ammoniain this instance is the precipitation of nickel ammonium sulphate, morecommonly known as nickel double salts, when dealing with solutionscontaining high nickel and ammonium sulphate concentrations. For thisreason, the feed solution is usually diluted to 50-70 g/L nickelstrength to prevent the formation of nickel double salts.

[0003]FIG. 1 is a conventional process flowsheet for refining a mixedsulphide showing the extraction of cobalt from a nickel sulphatesolution with a direct addition of ammonia to a cobalt solventextraction circuit. The direct addition of ammonia to the concentratednickel sulphate solution results in the formation of insoluble nickelammonium sulphate double salts.

[0004] Australian patent No. 667539 by Outokumpu describes a two stageprocess which avoids the formation of this double salt by:

[0005] (i) pre-neutralisation of a cationic extractant such as Cyanex272 to form the ammonium salt; and

[0006] (ii) pre-extraction or exchange of the Cyanex 272 ammonium saltwith magnesium sulphate in an aqueous solution to form a Cyanex 272magnesium salt which is contacted with an aqueous nickel sulphatesolution in a solvent extraction circuit so as to extract nickel.

[0007] Another means of avoiding double salt formation is described inthe specification of the applicant's International patent applicationNo. PCT/AU98/00457. This avoids the relatively expensive two stagepre-equilibration proposed by patent No. 667539 by adding chemicallyreactive magnesia, magnesium hydroxide, or magnesium carbonate to thecationic extractant without the pre-neutralisation step. However, ifmagnesia or magnesium pre-equilibrated extractant is used to avoid theformation of double salts this introduces magnesium ions thatcontaminates the final ammonium sulphate product.

SUMMARY OF THE INVENTION

[0008] According to the present invention there is provided a method ofremoving impurity metals from an impure concentrated valuable metalsulphate stream in a solvent extraction circuit, said method involvingcontacting the impure concentrated valuable metal sulphate stream with acationic solvent extractant, in the solvent extraction circuit operatedat a relatively high temperature which is effective in increasing thesolubility of the valuable metal in the concentrated sulphate streamcontaining ammonium sulphate, said extraction circuit also beingoperated whereby one or more of the impurity metals is loaded on thecationic solvent extractant using ammonia to control the pH whilst araffinate of the solvent extraction circuit which contains the valuablemetal is enriched in ammonium sulphate.

[0009] Conventionally when ammonia serves as the neutralising agent withconcentrated nickel sulphate liquors, insoluble nickel salts such asnickel ammonium sulphate double salts may be formed. According to anembodiment of the invention, the formation of insoluble valuable metalsalts is avoided by operating the solvent extraction circuit atsignificantly higher temperatures than is conventionally practised, andby using kerosene diluents with significantly higher flash pointtemperatures.

[0010] Generally the impure valuable metal sulphate stream is a nickelsulphate liquor, for example that obtained by acid/oxygen pressureleaching of a nickel-cobalt sulphide concentrate or a mixednickel/cobalt sulphide precipitate obtained during the processing ofnickel lateritic ores.

[0011] Preferably the relatively high temperature is greater than about60° C. More preferably the high temperature is between about 80 to 100°C.

[0012] Typically the nickel sulphate liquor is a relatively concentratedliquor. More typically the nickel sulphate liquor contains at leastabout 60 g/L Ni.

[0013] Preferably the cationic solvent extractant is mixed with anorganic diluent of a relatively high flash point. More preferably theorganic diluent is a paraffin based diluent such as that commerciallyavailable as ISOPAR V.

[0014] Typically the solvent extraction circuit is designed to removeone or two of said impurity metals, respectively, from the nickelsulphate liquor. More typically two extraction circuits are designed toremove Zn and Co, respectively, from the nickel sulphate liquor.

[0015] Preferably the cationic solvent extractant comprises a phosphinicacid such as a bis (2,4,4 trimethylpentyl) phosphinic acid, for examplethat commercially available as CYANEX 272.

[0016] Generally the impurity metals include Co, Zn, Fe, Al, Cr and Cu.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to achieve a better understanding of the nature of thepresent invention, a preferred embodiment of a method of removingimpurity metals from an impure concentrated valuable metal sulphatestream in a solvent extraction circuit will now be described, by way ofexample only, with reference to the following drawings:

[0018]FIG. 1 is a conventional refinery flow sheet for treating nickelsulphate leach liquors;

[0019]FIG. 2 is a two stage zinc/cobalt solvent extraction recoverycircuit;

[0020]FIG. 3 is a pH extraction isotherm at a temperature of 85° C. forCyanex 272 in a preferred high flash point temperature diluent using animpure concentrated nickel sulphate liquor;

[0021]FIG. 4 is a comparative pH extraction isotherm at 50° C. forCyanex 272 in Shellsol 2046 kerosene diluent for an impure nickelsulphate liquor; and

[0022] FIGS. 5 to 8 are nickel solubility plots at 50° C., 60° C., 70°C. and 80° C. which show the relative solubility of a range of nickelsulphate solutions in the presence of various ammonium sulphateconcentrations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] According to one embodiment of a process for the solventextraction of impurity metals from a concentrated nickel sulphate liquorthe solvent extraction circuit is operated at a temperature exceeding60° C. being the maximum temperature at which solvent extractioncircuits conventionally operate. This is now feasible with theavailability of high flash point diluents and the commercialisation ofpulse columns. This will for example enable the conventional processflowsheet shown in FIG. 1 to be operated at high nickel strengths andavoid dilution of the nickel process stream prior to cobalt removal. Ithas been found in one embodiment that by operating the solventextraction circuit at 85° C. ammonium sulphate concentrations above 50g/L can be tolerated at nickel strengths of 90-100 g/L.

[0024] A cationic solvent extractant Cyanex 272 (a phosphinic acid), wasused in this example together with an isoparaffin diluent ISOPAR V. Animpure valuable metal sulphate stream in this example is a nickelsulphate liquor which was produced from pressure leaching a mixedsulphide which in turn had been produced by treating a nickel lateriteleach liquor with hydrogen sulphide. The liquor composition is shown inTable 1. TABLE 1 Impure Nickel Sulphate Solution Composition (g/L) Ni CoZn Cu Mn Fe Cr Ca Al S Amsul 103 8.78 0.634 0.002 0.012 0.246 0.0630.060 0.082 68.0 21.7

[0025] Cyanex 272 extractant is a phosphinic acid that is highlyselective in the separation of zinc and cobalt from nickel. When in itsprotonated form (H⁺) the pH decreases as the zinc or cobalt is extractedand exchanges with the hydrogen ion according to the following reaction:

2R₂POO⁻H⁺+M²⁺

(R₂POO⁻)₂M²⁺+2H⁺  (1)

[0026] where, M=Zn or Co.

[0027] The acid generated from the reaction is neutralised with ammoniato form ammonium sulphate.

[0028]FIG. 2 shows a two step solvent extraction circuit to removeimpurity metals of zinc and cobalt selectively from a valuable metal ofnickel. In order to selectively extract zinc there must be sufficientseparation between zinc and cobalt in the Cyanex 272 system. Similarlyfor cobalt from nickel, the separation factor must be of sufficientmagnitude to obtain a pure nickel product. The separation factor isdependent on a number of variables including temperature, aromaticcontent of diluent, and extractant concentration. In the case of nickelhydrogen reduction up to 2 g/L cobalt can be present in the nickelreduction feed without causing any significant technical issues. In thecase of nickel electrowinning less than 10ppm cobalt can be present inthe nickel catholyte to result in the production of off-spec LME nickel.

[0029] The extraction pH isotherms were constructed to determine theoptimum operating pH during extraction. FIG. 3 shows the pH extractionisotherm according to one embodiment of the invention obtained at atemperature of 85° C. for 0.85 M Cyanex 272 in ISOPAR V using aconcentrated feed solution containing about 103 g/L Ni and 9 g/L Co.FIG. 4 shows a comparative pH extraction isotherm for 0.45 M Cyanex 272in Shellsol 2046 obtained at 50° C. using the impure feed solutiondiluted to a nickel concentration of 65 g/L. As mentioned above theseparation factor of Cyanex 272 is dependent on the temperature of theextraction and the aromatic content of the diluent. The two extractionisotherms of FIGS. 3 and 4 indicate a shift to the left with an increasein temperature and lower aromatic content of the diluent according tothe preferred method of the invention.

[0030] The bench scale tests indicated that selective extraction of zincover cobalt is possible with Cyanex 272. It is envisaged that this stagewill be controlled between pH 2.00 and 2.25. A certain amount of zinccan be tolerated in the cobalt and nickel reduction feed. This zincwould be precipitated at the end of hydrogen reduction as a mixedsulphide. It is anticipated that any co-extracted cobalt will berecovered in the scrub circuit and recycled to extraction as shown inFIG. 3.

[0031] Cobalt can be selectively extracted from nickel in the pH rangeof 4.00 to 4.50. Any Fe, Cu, Zn, Mn and Mg remaining in the liquor willbe fully loaded onto the organic with the cobalt. However, theseimpurities should be present at very low levels.

[0032] FIGS. 5 to 8 are plots of the relative nickel solubility over arange of nickel concentrations in the presence of various concentrationsof ammonium sulphate at selected temperatures. FIGS. 5 and 6 areintended as comparative plots at the maximum normal operatingtemperatures for solvent extraction and show significant precipitationof nickel occurs once the concentration of Ni exceeds 60 g/L in thepresence of more than 40 g/L ammonium sulphate. FIGS. 7 and 8 give anindication of nickel solubility at temperatures within the scope of thepreferred embodiment of the present invention. It is clear that theincreased temperatures increase the solubility of the valuable metal, inthis example nickel, with relatively high concentrations of ammoniumsulphate. For example, at 80° C. an ammonium sulphate concentration of50 g/L can be tolerated at nickel strengths of between 80 to 120 g/L.

[0033] Now that a preferred embodiment of the invention has beendescribed in some detail it will be apparent to those skilled in the artthat the method of removing impurity metals in a solvent extractioncircuit has at least the following advantages:

[0034] (i) increased concentrations of nickel can be tolerated in thesolvent extraction circuit; and

[0035] (ii) relatively high temperatures for the extraction circuit arepossible with high flash point diluents.

[0036] Those skilled in the art will appreciate that the inventiondescribed herein is susceptible to variations and modifications otherthan those specifically described. For example, the impure valuablemetal sulphate stream may be cobalt sulphate and is not limited to theconcentrated nickel sulphate liquor described. Furthermore, thetemperature of the extraction circuit need not be restricted to thetemperatures described but rather extend to relatively high temperaturesthat are effective in increasing the solubility of the valuable metal inthe sulphate stream in the presence of added ammonium sulphate. Thecationic solvent extractant and diluent described may also vary providedthe required impurity metal loading is achieved.

[0037] All such variations and modifications are to be considered withinthe scope of the present invention the nature of which is to bedetermined from the foregoing description.

[0038] It is to be understood that a reference herein to a prior artdocument does not constitute an admission that the document forms partof the common general knowledge in the art in Australia or in any othercountry.

1) A method of removing impurity metals from an impure concentratedvaluable metal sulphate stream in a solvent extraction circuit, saidmethod involving contacting the impure concentrated valuable metalsulphate stream with a cationic solvent extractant, in the solventextraction circuit operated at a relatively high temperature which iseffective in increasing the solubility of the valuable metal in theconcentrated sulphate stream containing ammonium sulphate, saidextraction circuit also being operated whereby one or more of theimpurity metals is loaded on the cationic solvent extractant usingammonia to control the pH whilst a raffinate of the solvent extractioncircuit which contains the valuable metal is enriched in ammoniumsulphate. 2) A method as defined in claim 1 wherein the relatively hightemperature is greater than about 60° C. 3) A method as defined in claim1 wherein the high temperature is between 80 to 100° C. 4) A method asdefined in any one of the preceding claims wherein the impure valuablemetal sulphate stream is a nickel sulphate liquor. 5) A method asdefined in claim 4 wherein the nickel sulphate liquor is obtained byacid/oxygen pressure leaching of a nickel/cobalt sulphide precipitateobtained during the processing of nickel lateritic ores. 6) A method asdefined in claim 4 or 5 wherein the nickel sulphate liquor is arelatively concentrated liquor. 7) A method as defined in claim 6wherein the nickel sulphate liquor contains at least about 60 g/L Ni. 8)A method as defined in any one of claims 4 to 7 wherein the solventextraction circuit is designed to remove one or two of said impuritymetals, respectively, from the nickel sulphate liquor. 9) A method asdefined in claim 8 wherein two extraction circuits are designed toremove Zn and Co, respectively, from the nickel sulphate liquor. 10) Amethod as defined in any one of the preceding claims wherein thecationic solvent extractant is mixed with an organic diluent of arelatively high flash point. 11) A method as defined in claim 10 whereinthe organic diluent is a paraffin based diluent. 12) A method as definedin any one of the preceding claims wherein the cationic solventextractant comprises a phosphonic acid such as a bis (2,4,4trimethlypentyl) phosphonic acid. 13) A method as defined in any one ofthe preceding claims wherein the impurity metals include Co, Zn, Fe, Al,Cr and Cu.